Optical fibres with special bending and dispersion properties

ABSTRACT

A microstructured optical fiber having a specially designed cladding to provide single mode waveguidance and low sensitivity to bending losses. In one aspect the optical fiber has an inner and an outer cladding each comprising elongated features. The inner cladding features have normalized dimensions in the range from 0.35 to 0.50 and the outer cladding features have normalized dimensions in the range from 0.5 to 0.9, where the normalization factor is a typical feature spacing. The fiber is further characterized by a feature spacing of the inner cladding larger than 2.0 micron. In a second aspect, the fiber has a special non-circular and non-equilateral-polygonial outer cross-sectional shape to mechanically ensure bending in predetermined directions that are favourable with respect to low bending losses. The present invention provides fibers, which are less sensitive to macro-bending losses than presently known single-mode fibers with similar sized mode areas, and provides robust, single-mode, large-mode area fibers for long-distance optical transmission and fibers with special dispersion properties.

This is a nationalization of PCT/DK01/00746 filed Nov. 12, 2001 andpublished in English.

FIELD OF THE INVENTION

The present invention relates to electromagnetic waveguides, especiallyoptical fibres, having micro-structures in core and/or claddingregion(s).

BACKGROUND OF THE INVENTION

While transmission speeds are pushed into the Tbit range for modern,multi-channel telecommunication systems, a large interest is pointedtowards new optical fibres with relatively large mode areas. One of thekey issues is to avoid disturbance of individual channels, while theseare transmitted over a fibre link. The disturbance is mainly related tonon-linear effects in the fibres—effects that can be suppressed or inpractice eliminated by the use of fibres with large mode areas and/orspecial dispersion properties.

For many applications, a large mode area can, however, not be toleratedusing presently known fibres, since such fibres exhibit highmacro-bending losses. This is e.g. the case for applications wherefibres are used in compact modules and consequently must be coiled withrelatively small bending radil—typically around 6 cm—such as fordispersion compensating fibre modules. The present invention solves theproblem of macro-bending sensitive large-mode area fibres, and providesrobust, single-mode, large-mode area fibres for long-distance opticaltransmission and for fibres with special dispersion properties such asdispersion-shifted fibres, dispersion compensating and dispersion slopecompensating fibres. Typically, optical fibres for dispersioncompensation have a core diameter of around 4 μm, and the presentinvention provides fibres with core diameter larger than 4 μm.

Recently a new type of optical fibre that is characterized by aso-called micro-structure has been demonstrated. Optical fibres of thistype (which are referred to by several names—e.g. micro-structuredfibres, photonic crystal fibres, holey fibres, or photonic bandgapfibres) have been described in a number of references, such as WO99/64903, WO 99/64904, and Broeng et al. (see Pure and Applied Optics,pp.477-482, 1999) describing such fibres having claddings definingPhotonic Band Gap (PBG) structures, and U.S. Pat. No. 5,802,236, Knightet al. (see J. Opt. Soc. Am. A, Vol. 15, No. 3, pp. 748-752, 1998),Monro et al. (see Optics Letters, Vol.25 (4), p.206-8, February 2000)defining fibres where the light is transmitted using modified TotalInternal Reflection (TIR). This invention concerns mainly fibres thatare guiding by TIR.

Micro-structured fibres are known to exhibit waveguiding properties thatare unattainable using conventional fibres. One of these uniqueproperties is that micro-structured fibres can be designed to beso-called endlessly single mode (see Birks et al. Optics Letters, July1, 22(13), pp. 961-963, 1997). Such fibres have a very important aspect,namely that they, in principle, can be designed with a very large modearea at any desired wavelength while remaining single mode (see Birks etal. Electronics Letters, June 25, 34(13), pp. 1347-1348, 1998, and WO99/00685). Although endlessly single mode fibres known from the priorart may, in theory, have infinitely large mode areas, the fibres will inpractice have a mode area that is limited by macro-bending losses (seeSørensen et al. Electronics Letters, Vol.37, no.5, 1^(st) Mar. 2001 andSørensen et al. 27^(th) European Conference on Optical Communication,paper We.P.1, Amsterdam, 2001). As demonstrated in the literature, theprior art, endlessly single mode fibres must have a filling fraction ofthe micro-structured cladding that is lower than a certain criticalvalue. Often the micro-structured cladding is formed using periodicallyarranged air holes in a silica background material, as for the fibres inthe above-cited references. The critical air filling fraction of priorart endlessly, single mode micro-structured fibre designs has beeninvestigated both experimentally by Knight et al. (see the above-citedKnight reference) and theoretically by Broeng et al. (see Broeng et al,Optical Fiber Technology, Vol. 5, pp.305-330 1999). In theory, forfibres made from pure silica, having circular air holes placed in aclose-packed arrangement—or triangular arrangement—a critical fillingfraction of 18% has been found. In practice, for prior art fibres alower filling fraction—of less than 5%—has been found. Hence, fibresknown from the prior art are not able to be endlessly single-mode (and,therefore, to have a very large mode area) unless the filling fractionis below at least 18%. For close-packed arrangement of air holes insilica, the critical air filling fraction of 18% corresponds to a holediameter, d, of around 0.45 times the center-to-centre spacing, Λ, of tonearest air holes.

The present inventors have realized that a more appropriate parameterwith respect to the cut-off properties of micro-structured fibres, isthe minimum spacing, w, between boundaries of two nearest holes. Bygeometrical considerations, w may be deduced to be equal to Λ−d. Hence,the above-mentioned critical air filing fraction of 18%—or critical holediameter of 0.45Λ—corresponds to a critical (minimum) w value of 0.55Λ.

The critical filling fraction is an important parameter for practicalapplications, as it is one of the key parameters that determine therobustness of a micro-structured fibre in terms of low macro-bendinglosses. A large filling fraction is a general requirement in order toeliminate macro-bending losses.

It is a disadvantage of single-mode, micro-structured fibres with corediameters above 4 μm, which are known in the prior art, that they havefilling fractions of 18% or smaller. The present invention discloses anumber of new designs of relatively large mode area, single mode fibreswith larger critical air filling fraction than known from the prior art,as well as a method for producing fibres with such designs. This isachieved be using two concentric cladding regions—an inner and an outercladding region—where the outer cladding region has a filling fractionlarger than 18% and the inner cladding region is designed to have waround 0.55Λ_(i), where Λ_(i) is the center-to-centre spacing of twonearest features in the inner cladding. Hence, in the case of both theinner and outer cladding features being circular air holes, the innercladding features have a diameter, d_(i), of around 0.45Λ_(i), whereasthe outer cladding features have a diameter, d_(o), larger than0.50Λ_(o) (Λ_(o) being the center-to-centre spacing of two nearestfeatures in the outer cladding). The larger filling fraction in theouter cladding provides improved macro-bending losses properties,whereas the design of the inner cladding features ensures single modeoperation. Hence, the present invention provides greater flexibilitywhen designing fibres having relatively large mode areas (larger than 4μm in core diameter) with respect to macro-bending losses and/ordispersion properties. Fibres according to the present invention will beless sensitive to macro-bending losses than presently known single-modefibres with similar sized mode areas—due to the special relation betweenthe dimensions of the inner and outer cladding features. Furthermore,single-mode fibres according to the present invention may be utilizedfor applications, where a relatively large mode area and specificallytailored dispersion properties are of importance, such as dispersioncompensating fibres and dispersion slope compensating fibres. The fibresare, however, not only of interest in optical telecommunication, butalso for very high power light transmitting systems, such as e.g. lasermachining and medical surgery, as well as the fibres are of interest forsensing systems.

GLOSSARY AND DEFINITIONS

In this application we distinguish between “refractive index” and“effective refractive index”. The refractive index is the conventionalrefractive index of a homogeneous material. In this application, weconsider mainly optical wavelengths in the visible to near-infraredregime (wavelengths from approximately 300 nm to 2 μm). In thiswavelength range most relevant materials for fibre production (e.g.silica) may be considered mainly wavelength independent, or at least notstrongly wavelength dependent. However, for non-homogeneous materials,such as micro-structures, the effective refractive index is verydependent on the morphology of the material. Furthermore, the effectiverefractive index of a micro-structure is strongly wavelengthdependent—much stronger than the refractive index of any of thematerials composing the micro-structure. The procedure of determiningthe effective refractive index of a given micro-structure at a givenwavelength is well-known to those skilled in the art (see e.g.Jouannopoulos et al, “Photonic Crystals”, Princeton University Press,1995 or Broeng et al, Optical Fiber Technology, Vol. 5, pp.305-330,1999). The present invention takes advantage of specific micro-structuremorphologies, their strong wavelength dependency, and proper core designin a way to realize large filling fraction, single mode fibres. Afurther useful parameter is the geometrical index, which may very simplybe determined from direct inspection of the cross-sectional morphologyof a micro-structured fibre and knowledge of the refractive indices ofthe materials that compose the micro-structure. Hence, a silica-airmicro-structured region with an air filling fraction of 18% (for examplea region with air holes having a diameter of 0.45Λ) will have ageometrical index of 0.18*1.0+0.82*1.45 equal to 1.369 (where therefractive indices of air and silica have been assumed equal to 1.0 and1.45, respectively).

Usually a numerical method capable of solving Maxwell's equation on fullvectorial form is required for accurate determination of the effectiverefractive indices of micro-structures. The present inventors make useof such a method that has been well-documented in the literature (seeprevious Joannopoulos-reference). In the long-wavelength regime, theeffective refractive index is roughly identical to the weighted averageof the refractive indices of the constituents of the material. Formicro-structures, a directly measurable quantity is the so-calledfilling fraction, f, that is the volume of disposed features in amicro-structure relative to the total volume of that micro-structure. Ofcourse, for fibres that are invariant in the axial fibre direction, thefilling fraction may be determined from direct inspection of the fibrecross-section.

SUMMARY OF THE INVENTION

The main problem solved by the present invention is to guide light insingle-mode micro-structured fibres, while having a large-mode area andnegligible macro-bending losses. The fibres are aimed at applications atvisible to near-infrared wavelengths. Large features (usually in theform of air holes) in the cladding of micro-structured fibres providegenerally lower macro-bending losses and it is, therefore, desirable torealize micro-structured fibres with large features. The main problemfor the fibre designs disclosed in the prior art is, however, that abovea certain air hole size, the fibres may become multi-mode. To thoseskilled in the art it is well known that prior art micro-structuredfibres with close-packed air holes in a silica cladding and a singlemissing air hole to form the core require small cladding features (afilling fraction below 18%) in order to obtain single-mode operation forlarge core sizes.

It is a disadvantage of single-mode, micro-structured fibres with corediameters above 4 μm, which are known in the prior art, that they havefilling fractions of 18% or smaller. The present invention discloses anumber of new designs of relatively large mode area, single mode fibreswith larger critical air filling fraction than known from the prior art,as well as a method for producing fibres with such designs. This may beachieved be using two concentric cladding regions—an inner and an outercladding region—where the outer cladding region has a filling fractionlarger than 18% and the inner cladding region is designed to have waround 0.55Λ_(i), where Λ_(i), is the center-to-centre spacing of twonearest features in the inner cladding. Hence, in the case of both theinner and outer cladding features being circular air holes, the innercladding features may have a diameter, d_(i), of around 0.45Λ_(i),whereas the outer cladding features may have a diameter, d_(o), largerthan 0.50Λ_(o) (Λ_(o) being the center-to-centre spacing of two nearestfeatures in the outer cladding). The larger filling fraction in theouter cladding may provide improved macro-bending loss properties,whereas the design of the inner cladding features ensures single modeoperation. Hence, the present invention may provide greater flexibilitywhen designing fibres having relatively large mode areas (larger than 4μm in core diameter) with respect to macro-bending losses and/ordispersion properties. Fibres according to the present invention will beless sensitive to macro-bending losses than presently known single-modefibres with similar sized mode areas—due to the special relation betweenthe dimensions of the inner and outer cladding features. Furthermore,single-mode fibres according to the present invention may be utilizedfor applications, where a relatively large mode area and specificallytailored dispersion properties are of importance, such as dispersioncompensating fibres and dispersion slope compensating fibres. The fibresare, however, not only of interest in optical telecommunication, butalso for very high power light transmitting systems, such as e.g. lasermachining and medical surgery, as well as the fibres are of interest forsensing systems.

According to a first aspect of the invention, there is provide amicro-structured optical fibre for guiding light at an operatingwavelength, said optical fibre having an axial direction and a crosssection perpendicular to said axial direction, said optical fibrecomprising:

-   -   a core region;    -   an inner cladding region surrounding said core region, said        cladding region comprising a multiplicity of spaced apart inner        cladding features that are elongated in the fibre axial        direction and disposed in an inner cladding material, said inner        cladding features having a smallest cross-sectional dimension,        d_(i,min), and a largest cross-sectional dimension, d_(i,max),        and having a centre-to-centre spacing, Λ_(i), between two        neighbouring inner cladding features;    -   an outer cladding region surrounding said inner cladding region,        said outer cladding region comprising a multiplicity of spaced        apart outer cladding features that are elongated in the fibre        axial direction and disposed in an outer cladding material, at        least part of said outer cladding features having a smallest        cross-sectional dimension, d_(o,min), and having a        centre-to-centre spacing, Λ_(o), between two neighbouring inner        cladding features; wherein d_(i,min) is in the range from        0.35Λ_(i) to 0.60Λ_(i), d_(o,min) is in the range from 0.50Λ_(o)        to 0.90Λ_(o), and Λ_(i) is larger than 2 μm.

When in the present context reference is made to the “axial direction”and the “cross section” or the “cross-sectional dimension” in relationto the optical fibre and features therein (the features being located inthe cladding or in the core), the cross section is to be taken to meanthe cross section that is perpendicular to an axial line of the fibrewhen the fibre is straight. It is preferred that the cross section issubstantially identical throughout the length of the optical fibre.

According to a first preferred embodiment of the invention, d_(i,min) issmaller than d_(o,min). Here it is also preferred that d_(i,min) shouldbe in the range from 0.35Λ_(i) to 0.50Λ_(i).

According to a second embodiment of the invention, Λ_(i) is larger thanΛ_(o).

In a preferred embodiment, d_(i,min) may be in the range from 0.40Λ_(i)to 0.45Λ_(i) or about 0.45Λ_(i). It is also preferred that d_(o,min) maybe in the range from 0.50Λ_(o) to 0.60Λ_(o).

The inner cladding features may have different shapes, includingsubstantially circular and elliptical, but it is preferred thatd_(i,max) is in the range from 1.0d_(i,min) to 2.0d_(i,min). It is alsowithin a preferred embodiment that d_(i,max) is larger than 0.45 timesthe core radius. In another preferred embodiment d_(i,max) is largerthan 0.45Λ_(i), such as larger than 2Λ_(i).

For the centre-to-centre spacing of the cladding features, it ispreferred that Λ_(i) is in the range from 0.3Λ_(o) to 3.0Λ_(o), or Λ_(i)is about equal to Λ_(o).

Here it should be understood that according to the invention, the innercladding features may have substantially equal cross-sectionaldimensions, or they may have different cross-sectional dimensions.However, the smallest cross-sectional dimension should be within theinterval for d_(i,min), the largest cross-sectional dimension should bewithin the interval for d_(i,max), and the centre-to-centre spacing forthe inner cladding features should be larger than 2.0 μm. It should alsobe understood that for the part of the outer cladding features having asmallest cross-sectional dimension given by the interval for d_(o,min),these outer cladding features may have substantially equalcross-sectional dimensions, or they may have different cross-sectionaldimensions, as long as the smallest cross-sectional dimension is withinthe interval for d_(o,min).

The first aspect of the invention also includes embodiments in whichΛ_(i) is larger than 2.5 μm, such as larger than 3 μm, such as largerthan 5 μm, such as larger than 10 μm, or such as larger than 25 μm.

According to the first aspect of the invention it is preferred that theat least part of the outer cladding features have a largercross-sectional dimension than the inner cladding features. Thus, it ispreferred that d_(o,min) is between 10-50% larger than d_(i,min), suchas d_(o,min) being between 10-20% larger than d_(i,min), or such asd_(o,min) being between 10-15% larger than d_(i,min).

It is preferred that the optical fibre of the first aspect of theinvention is dimensioned for guiding light through total internalreflection or modified total internal reflection. Thus, when looking atthe geometrical indices it is preferred that the core region has ageometrical index N_(coge), the inner cladding region has a geometricalindex N_(ige), the outer cladding region has a geometrical indexN_(oge), and N_(coge)>N_(ige)> or =N_(oge) for light guided at theoperating wavelength. Similarly, when looking at the effectiverefractive indices, it is preferred that the core region has aneffective refractive index N_(coef), the inner cladding region has aneffective refractive index N_(ief), the outer cladding region has aneffective refractive index N_(oef), and N_(coef)>N_(ief)> or =N_(oef)for light guided at the operating wavelength.

It should be understood that the outer cladding region may comprisecladding features of different cross-sectional shapes and/or areas.However, it is preferred that the outer cladding features being nearestneighbours to the inner cladding region all have a smallestcross-sectional dimension given by d_(o,min). It is also within apreferred embodiment that a majority or all of the outer claddingfeatures have a smallest cross-sectional dimension given by d_(o,min).It is preferred that part of or all of the outer cladding features havea substantially circular cross-sectional shape.

The present inventors have realized that the critical filling fractionmay be increased by distributing cladding features (typically air holes)around the core with a non-circular shape, where the features have alargest dimension that is substantially directed towards the core centre(the cladding features are elongated, each having substantially an axisof symmetry that is directed towards the core centre). In this mannerthe bridging width, w, may be kept approximately equal to 0.5Λ_(i) whilethe air filling fraction is increased above the level of a close-packedstructure with similar value of w. This may ensure single mode operationfor fibres according to the present invention while a larger fillingfraction is obtained. Thus, the inner cladding features may have anon-circular shape with d_(i,max) being larger than d_(i,min), and it iswithin an embodiment of the invention that at least part of or all ofthe inner cladding features in the cross-section have an elongatedshape, with the largest cross-sectional dimension d_(i,max) beingoriented in a direction substantially towards the centre of the coreregion. Here it is preferred that the largest dimension is at least 10%larger than the smallest dimension. The largest dimension may also be atleast 50% larger than the smallest dimension.

According to a preferred embodiment, the individual elongated featuresmay have a substantially two-fold symmetry, and the largest and smallestdimensions each define an axis with these two axes being substantiallyorthogonal.

When having elongated innermost cladding features, it may be moreappropriate to describe the core using an inner dimension—defined as thesmallest dimension through the core centre that does not touch theinnermost cladding features. In preferred embodiments, the innercladding dimension is at least 2.0 μm, and preferably up to at least25.0 μm. In other preferred embodiments, the inner core dimension may besmaller than the inner cladding feature centre-to-centre spacing Λ_(i).This may improve the cut-off properties of the fibres.

The first aspect of the invention may cover embodiments, in which thecladding features are periodical features, but it may also coverembodiments in which all or at least a part of the cladding features arenon-periodical features. Thus, the present invention includesmicro-structured fibres, where the cladding features, which may beelongated features, may be either non-periodically or periodicallydistributed. Hence, when we are discussing the spacing of the claddingfeatures, we will mean the representative centre-to-centre distancebetween two neighbouring features. For periodically distributedfeatures, this centre-to-centre spacing is easily determined, and ise.g., for a close-packed arrangement of features identical to the pitchof the periodic structure. For non-periodic distributions, thecentre-to-centre spacing should be taken as the average centre-to-centredistance between neighbouring features in the relevant region. Forspecial distributions, e.g., in the case of a very low number offeatures, the centre-to-centre spacing should be taken as arepresentative centre-to-centre distance between neighbouring featuresin the relevant region.

The present inventors have further realised that micro-structured fibresmay be improved by providing a non-circular outer cladding tomechanically ensure bending in certain preferred directions. Theorientation of the outer cladding should preferably depend on thespecific micro-structuring of the central part of the fibre (the coreand inner cladding region). Thus, according to a second aspect of thepresent invention, there is provided a micro-structured optical fibrefor guiding light at an operating wavelength, said optical fibre havingan axial direction and a cross section perpendicular td said axialdirection, said optical fibre comprising:

-   -   a core, and    -   a cladding region surrounding said core region, said cladding        region comprising a multiplicity of spaced apart cladding        features that are elongated in the fibre axial direction and        disposed in a cladding material,    -   wherein the cross-sectional outer shape of the optical fibre has        a non-circular and non-equilateral-polygon form.

Such an outer shape may be of advantage for controlling polarization,macro-bending, and/or dispersion properties.

Here, the cladding region may comprise an outer cladding regionsurrounding an inner cladding region, with the inner cladding regionsurrounding the core region. It is preferred that the outer shape of theoptical fibre is formed by the cladding region surrounding the coreregion or by the outer cladding region surrounding the inner claddingregion. It is also preferred that at least part of or all of themultiplicity of spaced apart cladding features are arranged in the innercladding region.

According to an embodiment of the second aspect of the invention, theouter shape may be characterized by a largest cross-sectional outerdimension and a smallest cross-sectional outer dimension, and saidlargest outer dimension may be at least 10% larger than said smallestouter dimension. Here, the largest outer dimension may be larger than 80mm, such as larger than 125 mm.

According to the second aspect of the invention, the cross-sectionalouter shape may have different shapes, but it is preferred that thecross-sectional outer shape of the fibre has a substantially ellipticalform. However, it is also within the invention that the cross-sectionalouter shape of the fibre has a substantially rectangular form.

In a preferred embodiment of the second aspect of the invention, thesmallest outer dimension defines a first axis, where said first axissubstantially coincides with an axis through centres of two innermost,cladding features positioned on opposite sides of a core centre, orwhere said first axis is substantially parallel to a second axis throughcentres of two innermost, cladding features positioned on opposite sidesof a core centre. The present inventors have realized that such anorientation of a non-circular outer cladding region with respect to theorientation of the micro-structure around the core region may reducemacro-bending losses of micro-structured fibres.

In another preferred embodiment of the second aspect of the invention,the smallest outer dimension defines a first axis, and said first axisis substantially orthogonal to a third axis through centres of twoinnermost, cladding features positioned on opposite sides of a corecentre. The present inventors have realized that such an orientation maystrip off higher order modes efficiently and expands the single moderange of micro-structured fibres.

The second aspect of the invention covers embodiments in which thecladding region or the inner cladding region comprises a first and asecond sub-region with one or both of said sub-regions comprising anumber of cladding features, and wherein the core features of the firstsub-region in the cross section occupies a part of the first sub-regionthereby defining a first filling fraction, and the core features ofsecond sub-region in the cross section occupies a part of the secondsub-region thereby defining a second filling fraction, said firstfilling fraction being different to the second filling fraction. Thismay provide fibres with special bending properties, whereelectromagnetic energy (light) radiated away from the fibre core duringbending is directed in predetermined direction(s).

Here, one of the two sub-regions may have a filling fraction of morethan 18%, and the other sub-region has a filling fraction of less than18%. This may provide fibres that are single mode and macro-bendinginsensitive even for large mode areas, and small bending radil. Suchfibres displays special dispersion properties, that may actively betuned through tuning of the bending radius.

The second aspect of the invention also provides embodiments in whichthe cladding region or the inner cladding region comprises foursub-regions, with at least one or all of said sub-regions comprising anumber of cladding features, whereby for each sub-region a fillingfraction is defined by the part of the sub-region being occupied in thecross section by cladding features. This may further improve thepossibility of realizing single-mode, large-mode areas fibres that areinsensitive to macro-bending. Here, two of the four subregions may havea filling fraction of more than 18%, and the other two sub-regions havea filling fraction of less than 18%.

For embodiments with cladding sub-regions, it is preferred that acladding sub-region having a lower filling fraction comprises a numberof cladding features having a smaller cross-sectional area than thecross-sectional area of the cladding features of a sub-region with ahigher filling fraction. Here, when the smallest outer dimension definesa first axis, the first axis may substantially divide the claddingsub-region(s) having a lower filling fraction into two substantiallyequally sized halves. The present inventors have realized that such anorientation of the non-circular, outer cladding region with respect tothe micro-structure in the fibre, is optimum for certain fibres, such asfibres with direction-dependent radiation during fibre bending. In afurther preferred embodiment, the first axis is substantially dividingthe cladding sub-region(s) having a larger filling fraction in halves.The present inventors have realized that such an orientation of thenon-circular, outer cladding region with respect to the micro-structurein the fibre, provides lowest possible macro-bending losses formicro-structured fibres.

For certain applications, it is an advantage that the core material (thebackground material of the core region) has a different refractive indexthan the cladding material. This allows flexibility of tailoring thedispersive properties of the fibres. This refractive index differencemay be obtained e.g., by using different dopants in the two materials(e.g. silica doped to various degrees), different glasses, or it may beobtained simply by using different basis materials (e.g., differenttypes of polymers).

For both the first and the second aspects of the invention, it ispreferred that the core region has an effective refractive index beingis higher than the effective refractive index of the cladding region.This may ensure effective index guidance, also known as (modified) totalinternal reflection, for the guided light. Preferably, the core regionis a substantially solid core made of a core material. Here, theeffective refractive index of the core N_(coef) and/or the geometricalindex of the core N_(coge) may be substantially equal to the refractiveindex of the core material.

Different materials and configurations may be used in order to obtain acore region having an effective refractive index being is higher thanthe effective refractive index of the cladding region. Thus, the aspectsof the present invention cover embodiment in which the refractive indexof the core material is lower than the refractive index of the innerand/or outer cladding material. This may allow further manipulation ofthe cut-off properties of the fibres. Such fibres need not be endlesslysingle mode, but will suppress the second-order mode cut-off compared toa fibre with the same core and cladding background material, but anotherwise identical design.

However, in other preferred embodiments, the refractive index of thecore material is substantially equal to the refractive index of theinner and/or outer cladding material. This may e.g., be preferred incases, where absorption losses are a critical issue, and the fibre mustbe fabricated from the purest possible material. In this case it ispreferred to use the same material, which may be a pure material, forthe core material, the inner cladding material and/or the outer claddingmaterial. Also with respect to fabrication method, it may be anadvantage to use the same core and cladding material (and, therefore,the same refractive index of the core and cladding material). This ise.g., the case, where a difference in thermal expansion coefficient forthe core and cladding materials cannot be tolerated. The presently usedfabrication methods for micro-structured fibres are generally not infavour of the use of different core and cladding materials. Hence,fibres with the same core and cladding material are preferred.

According to preferred embodiments of the aspects of the invention, thecore material may be made of silica or polymer. It is also preferredthat the inner and/or outer cladding material is made of silica orpolymer.

However, the aspects of the invention also cover embodiments in whichthe core and/or any of the cladding materials comprise a dopant (e.g. anactive or photosensitive material) or a material showing higher order(non-linear) optical effects such as an increased third-ordernon-linearity. Such preferred embodiments allows the realization ofvarious applications, such as fibre laser, amplifiers, wavelengthconverters, optical switches etc. Higher order (non-linear) effects maybe used for e.g., soliton communication or more generally inapplications, where non-linear effects are influencing the propagationproperties of signals in optical communication systems. This alsoincludes realisation of components for optical signal processing and forswitching. Especially for applications for fibre lasers or fibreamplifiers, the dopant in the core or the cladding may be e.g., arare-earth dopant adapted to receive pump radiation and amplifyradiation travelling in the core region. The dopant may also be a lightsensitive dopant, such as Germanium. In that situation, the dopant maybe used for e.g., optically writing a grating in the fibre or coreregion. Of particular interest is the use of photosensitive materials toallow writing of 1D gratings in the longitudinal direction of thefibres. Fibres with such gratings, combined with the large mode area,are very attractive for high power fibre lasers.

For the different aspects of the invention it is preferred that thecladding features are voids. Here, the voids of the cladding regions maycontain air, another gas, or a vacuum.

Although it is preferred that the core region is substantially solid,the aspects of the invention also cover embodiments in which the coreregion comprises one core feature or a multitude of spaced apart corefeatures. This may allow for an even higher flexibility for increasingthe critical cladding filling fraction. Here, the core features may havea cross-sectional dimension being smaller than the smallestcross-sectional dimension of the inner and/or outer cladding features.According to embodiments of the invention, the core features may have acircular or non-circular symmetric shape in the fibre cross-section. Itis preferred that the core features are voids, and the voids of the coreregion may contain air, another gas, or a vacuum. However, it is alsowithin the aspects of the invention that at least part of or all of thecore features and/or the cladding features are voids containing orfilled up with a material having a refractive index that differs fromthe refractive index of the region containing the feature such aspolymer(s), a material providing an increased third-order non-linearity,a photo-sensitive material, or a rare earth material (including in theform of dopants present in a host matrix as rare earth ions).

It should be understood that although it is preferred that the coreregion has a substantially circular cross-section, it is also within theaspects of the invention that the shape of the core region deviatessubstantially from a circular shape in the fibre cross-section. Thus,the shape of the core region may be substantially rectangular in thefibre cross-section. However, the shape of the core region may also oralternatively deviate substantially from a quadratic shape, a hexagonalshape, or a higher order polynomial shape in the fibre cross-section.

According to preferred embodiments of the aspect of the invention, thecore region and/or the cladding regions may have substantially a 180degree rotational symmetry in the fibre cross-section.

Fibres according to the aspects of the present invention are intendedfor use in a wide range of applications, where the light guided throughthe fibre will be in the range from about 0.3 μm to 2 μm and the lightshould be guided in a single mode. Thus, it is preferred that theoptical fibre is dimensioned to guide light at an operating wavelengthselected from wavelengths in the range of 0.3 μm to 2 μm, such as in therange of 0.3 μm to 1.6 μm in a single mode. Preferably, the opticalfibre is dimensioned to guide light at an operating wavelength about 1.5μm in a single mode.

For use in certain systems, the operating wavelength may be veryshort—typically in the interval from 0.3 μm to 0.6 μm. For otherapplications, the fibre may be desired for delivery of light from lasersources such as III-V semiconductor lasers—with a wavelength range fromaround 0.6 μm to 1.6 μm. Particularly, the wavelength range around 0.8μm is of interest for delivery of light from relatively cheap GaAs basedsemiconductor lasers. For other applications, fibres according to thepresent invention may be used for applications such as delivery of lightfrom powerful, tuneable Tl:Sapphire lasers. Hence, the fibres may bedesigned to guide light at wavelengths between 0.78 μm to 0.98 μm. Forother systems, e.g., systems employing lasers and amplifiers based onrare-earth doping, the fibres may be desired to guide light at specificwavelengths, corresponding to transitions for particular rare-earths.Important transition lines are located around 1.06 μm and 1.55 μm. Thefibres according to the present invention may be used for a number oftelecommunication applications—e.g., for long-distance transmission,dispersion compensation or dispersion slope compensation—where the fibrewill predominantly be used in the wavelength range from about 1.2 μm to1.6 μm. Particularly, the fibres may find use in the so-called secondand third telecommunication window, i.e., for wavelengths around 1.3 μmand for wavelengths from around 1.5 μm to 1.6 μm. For yet otherapplications, the fibres may find use at mid-infrared wavelengths, suchas around 2.0 μm. The present invention covers preferred embodiments,where the operating wavelength is within the above-mentioned wavelengthranges.

For wavelength multiplexing applications used in telecommunicationsystems, it is preferred that the fibre is designed to be single mode atwavelengths in the range from 1.5 μm to 1.6 μm (or broader). For otherapplications, such as fibre amplifiers or fibre laser, the fibres aredesired to be single-mode at a pump wavelength that may be significantlybelow this wavelength range. Therefore, preferred embodiments of thepresent invention covers fibres with single mode operation forwavelength ranges down to 0.3 μm.

According to preferred embodiments of the aspects of the invention, thecore has a diameter larger than 10 μm. For future telecom applications acore size in the range from about 10 μm to 30 μm is desired. Forhigh-power applications, a larger core size is desired such as fromabout 10 μm to 50 μm.

In order to obtain a strongly dispersive waveguide characteristic, thepresent inventors have realized how to utilize that the cladding regioncan be designed to have a higher effective refractive index than thecore at wavelengths shorter than a normal operating wavelength, wherethe effective refractive index of the core is larger than the effectiverefractive index of the cladding. This provides a cut-off for thefundamental mode in the core region at a so-called shifting wavelength,where the effective refractive indices are equal, but a very strongdispersion at the (longer) normal operating wavelength. Therefore, thepresent invention covers fibres where the effective refractive index ofthe cladding is larger than the effective refractive index of the corebelow a shifting wavelength, this shifting wavelength being shorter thanthe normal operating wavelength. The present inventors have realizedthat the dispersion is strongest close to the shifting wavelength, anddepending on the application of the fibres, the shifting wavelength may,therefore, be tailored to a specific value. In order to obtain thestrong dispersion at particular predetermined wavelengths, the presentinvention, therefore, covers preferred embodiments, where the shiftingwavelength is below 1.5 μm, where it is below 1.3 μm, below 1.06 μm,below 0.8 μm, below 0.6 μm, and below 0.4 μm.

For a range of applications, it is desirable to control the polarizationof light guided through the fibre. The present fabrication techniques ofmicro-structured fibre do not allow the complete elimination ofasymmetries in the fibre cross-section. As those skilled in the art willrecognize, this means that the fundamental mode of micro-structuredfibres will have two nearly degenerate polarization states. Formicro-structured fibres, where polarization effects are soughteliminated (the fibres are intended to have a low birefringence), wewill note the two polarization states as substantially non-degenerate.For micro-structured fibres where, on the other hand, polarizationeffects are desired, the non-degeneracy may be enhanced such that thebirefringence can reach levels of 10⁻⁵ and even higher, such as of atleast 10⁻³. We will note such fibres as having a fundamental modeconsisting of two substantially, non-degenerate polarization states. Fora long range of applications, such as e.g. for high precisionlithographic systems, it is desired to have fibres with highbirefringence. Therefore, in a further preferred embodiment, the presentinvention covers fibres that guide light in two substantially,non-degenerate polarization states. To quantify the splitting of thepolarizations states, the invention covers preferred embodiments, wherethe fibre birefringence is at least 10⁻⁵, such as at least 10⁻⁴, such asat least 10⁻³.

To control the degree of birefringence, it is preferred to have fibreswith a core region having either small or large degree of asymmetry.This asymmetry may be obtained either by positioning of the corefeatures in an asymmetric manner, in an otherwise symmetric core region,or by having an asymmetry in the actual shape of the core region.Naturally, combinations of the afore-mentioned cases may also beemployed. By asymmetry is here meant a deviation away from a circularsymmetric shape or away from a quadratic, a hexagonal or a symmetric,higher order polynomial shape. Hence, an elliptical shape will be ofinterest to control birefringence. The present invention, therefore,covers preferred embodiments with the above-described manners ofapplying asymmetry to the core region.

Fibres according to the present invention will often have a solidovercladding surrounding the micro-structured cladding and core regions.Typically, this overcladding will consist of silica having a higherrefractive index than the micro-structured cladding region in order tostrip off cladding modes.

In the above-described preferred embodiments, it has been assumed thatthe core and cladding features have a lower refractive index than thecore and cladding material (the background material of these regions).The reason for this is that micro-structured fibres commonly arefabricated such that the cladding features are voids such as air holes.Hence, the cladding features have a lower refractive index than thesurrounding background material.

In further preferred embodiments, fibres according to the presentinvention have a part of the core containing a higher refractive index.This is advantageous when splicing micro-structured optical fibres toother optical fibres. Thus, the aspects of the invention also coverembodiments in which the core region comprises a core part or a corefeature with a higher refractive index than any material surroundingsaid core part or feature. Here, the part of the core or the corefeature containing a higher refractive index may have a diameter of lessthan 2 μm, such as less than 1 μm.

In preferred embodiments, an optical fibre according to the presentinvention are used in compact modules, where the fibre is coiled to arelatively small radius, such as smaller than 20 cm, such as smallerthan 15 cm, or such as smaller than 10 cm. Due the improvedmacro-bending properties of fibres according to the present invention,the fibres may also be used at coiling radil around 6 cm or even lesssuch as around 1 cm or smaller.

In yet other preferred embodiments, an optical fibre according to thepresent invention is used for dispersion compensating and/or dispersionslope compensating applications in a telecommunication system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically the design of a typicalmicro-structured fibre known from the prior art.

FIG. 2 shows a scanning electron micrograph of a real, micro-structuredfibre with a design known from the prior art.

FIG. 3 shows the cut-off properties of a prior art, micro-structuredfibre.

FIG. 4 shows the mode field distribution of the fundamental mode of amicro-structured fibre.

FIG. 5 shows the mode field distribution of the second-order mode of amicro-structured fibre.

FIG. 6 illustrates the effective frequency, V, as a function ofnormalized frequency for a prior art fibre.

FIG. 7 illustrates the macro-bending losses of prior art fibres withd/Λ=0.4, 0.5, and 0.6.

FIG. 8 shows a micrograph of a large-mode area fibre realize using theprior art fabrication method (picture taken from Birks et al.Electronics Letters, June 25, 34(13), pp. 1347-1348, 1998).

FIG. 9 illustrates the effective frequency, V, as a function ofnormalized frequency for a series of fabricated prior art fibres.

FIG. 10 shows schematically the cross-section of a fibre according tothe present invention. The inner cladding features have a diameter of0.45Λ_(i) and the outer cladding features have a diameter of 0.55Λ_(o).

FIG. 11 shows schematically the cross-section of a fibre according tothe present invention. The inner cladding features have a diameter of0.45Λ_(i) and one part of the outer cladding features have a diameter of0.55Λ_(o) and a second part of the outer cladding features have adiameter larger than 0.6Λ_(o).

FIG. 12 shows schematically the cross-section of another fibre accordingto the present invention where the inner cladding features are orientedtowards the centre of the core.

FIG. 13 illustrates schematically the effective frequency, V, as afunction of normalized frequency for a series of fibres according to thepresent invention with a design as in FIG. 12. The second order mode cutoff is pushed to higher V values for fibres according to the presentinvention.

FIG. 14 shows a comparison between bending losses for three fibres withdifferent air filling fractions.

FIG. 15 shows schematically a possible fabrication method for realizingfibres according to the present invention. FIG. 15 a shows the fibrepreform and FIG. 15 b shows the final fibre.

FIG. 16 shows schematically another fibre according to the presentinvention.

FIG. 17 shows schematically a fibre according to the present inventionwhere a non-circular outer cladding is used. The orientation of theouter cladding is determined from the micro-structure in or around thecore region. The outer cladding shape determines mechanically whichbending direction(s) will be dominant for the fibre, and avoids bendingdirections that are most harmful for the fibre.

FIG. 18 shows schematically a fibre according to the present inventionwhere an outer cladding with a non-circular and non-equilateralpolygonal outer shape is used. During bending, this shape willmechanically favour bending of the fibre in certain direction comparedto others and thereby provide new means for controlling macro-bendinglosses properties as well as stripping off of higher order modes, and,thereby, expand the single-mode operation range of the fibre.

FIG. 19 shows schematically another fibre according to the presentinvention where an elliptical outer shape is used and the overcladdingis made from the same material as the background material of themicro-structure.

FIG. 20 shows schematically another fibre according to the presentinvention where a rectangular outer shape is used.

FIG. 21 shows schematically another fibre according to the presentinvention where two (smaller) circular, elongated elements have beenfused on the outside of the fibre to favour mechanically bending inpredetermined directions.

FIG. 22 shows schematically a fibre according to the present inventionwhere a non-circular and non-equilateral polygonal outer cladding isused. The orientation of the outer cladding is determined with respectto the orientation of the micro-structure in or around the core region.The outer cladding shape and the micro-structure in the fibre determinesa preferred direction of radiation during fibre bend. This fibre may,alternatively, be used to provide strongly dispersive fibres—where thedispersion can be tuned through adjusting the fibre bend radius.

FIG. 23 shows another example of a micro-structured fibre with anon-circular outer and non-equilateral polygonal cladding. The fibrewill be single mode and robust for larger mode areas than prior artfibres.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

In the proceeding text, the invention will be described by way ofexample using selected preferred embodiments.

In the prior art, micro-structured fibres with endlessly single modebehaviour have been based on a design as schematically shown in FIG. 1.The figure shows the cross-section of a fibre that is assumed invariantin the longitudinal direction. The fibre has a triangular arrangement ofair holes 100 in a silica background material 101 and a single missingair hole in the centre to form the core of the fibre 102. The outer partof the cladding region does not need to be micro-structured, and thisfigure shows an example of a fibre with a solid overcladding formed fromthe same material as the background material 101. The cladding holeshave a spacing Λ and a diameter d. The filling fraction of the fibre—orrather of the micro-structured part of the cladding—may be determineddirectly from an inspection of the fibre morphology. For periodicmicro-structures, the filling fraction, f, is directly related to d/Λ(for a triangular feature arrangement f is equal top/(2*sqrt(3))*(d/Λ)²). In the prior art, the core diameter has beendefined using the circular ring 103 (see WO 99/00685). We willpredominantly use this definition of the core diameter, although forcertain fibres it may be more relevant to describe the core using thecircular ring 104. The ring 104 is defined as the largest circular partof the core region that only contains high index material. When usingthe term high index material here, we mean in comparison with the indexof the cladding feature material(s). To describe the core using 104instead of 103 may be more relevant in the case of fibres with large airholes as well as it is more relevant for specific aspects of thisinvention, as will be discussed at a later stage in this application. Welabel the diameter of the ring 104 the inner diameter and it should benoticed that the inner diameter from 104 is always smaller than thediameter from 103. Unless otherwise stated, by core diameter, we willrefer to the core diameter defined using 103. For micro-structuredfibres with vanishing air hole sizes, the two diameters will be almostidentical, whereas for micro-structured fibres with a design as shown inFIG. 1 and largest possible circular, air holes (d_(i)=Λ_(i)), the innerdiameter from 104 will approximately be equal to half the diameter 103,hence the diameter 103 it will approximately be equal to Λ. It isfurther important to notice that for designs shown in the prior art oflarge mode area, micro-structured fibres, the inner diameter 104 issignificantly larger than Λ.

In order to fabricate micro-structured fibres, a number of capillarytubes and solid canes are stacked together to form a preform. Thispreform is then drawn into optical fibre using a conventional drawingtower. Usually, micro-structured fibres with a solid core, which areguiding light by use of TIR, are fabricated by way of a solid cane inthe centre of the preform to form the core. An example of a fibrerealised using the prior art fabrication technique is illustrated inFIG. 2. The present inventors have realised that it is not a requirementto fabricate high-index core micro-structured fibres using one or moresolid canes to form the core region. In contrast, the present inventorshave realized that it may be advantageous to form the core from one (ormore) capillary tubes that collapse completely during drawing of thefibre. Such a fabrication method may provide single-mode, large-modeareas fibres with lower macro-bending losses than the previously knownmicro-structured fibres, as shall be demonstrated later in thisdescription of the invention.

Before presenting fibres according to the present invention, it isnecessary to consider the cut-off properties of prior art,micro-structured fibres. FIG. 3 presents an analysis of a fibre with adesign as shown schematically in FIG. 1. The fibre has d/Λ=0.6 and it isseen only to be single mode up to normalized frequencies of around 1.5.We use normalized frequencies, Λ/λ, as the properties ofmicro-structured fibres may be scaled to a given wavelength range byscaling the dimension of the micro-structure. For a fibre with Λ=4 μm (acore diameter of 8 μm), the fibre will be multi-mode for wavelengthsshorter than 2.6 μm. Hence, for applications within the usualtelecommunications spectral ranges (near-infrared wavelengths between1.3 μm and 1.6 μm), this fibre will not be suitable (although it has arelatively large mode area).

The fundamental and the second-order mode of the fibre analysed in FIG.3 are shown in FIG. 4 and FIG. 5, respectively. As known, from theliterature, the fundamental mode of the micro-structured fibre closelyresembles the fundamental mode of a conventional fibre. Also thesecond-order mode resembles that of conventional fibres, having a 180degrees phase reversal between the two field lobes (indicated by theplus and minus signs).

A useful tool for the characterization of micro-structured fibres thathave been used extensively in the literature, and which we will also useto demonstrate some of the advantages of fibres according to the presentinvention, is an illustration of the so-called effective frequency, V,as a function of normalized frequency. This tool has e.g. been used byBirks et al. to demonstrate that micro-structured fibres may beendlessly single mode (see Birks et al. Optics Letters, July 1, 22(13),pp. 961-963, 1997). FIG. 6 shows the effective V for a series ofmicro-structured fibres with a design as shown in FIG. 1. As well knownfrom the literature, the fibres exhibit a second order mode cut-off ataround V=4.2 (using the effective V definition in the above-cited Birksreference). As is also well known from the literature, and as directlyobservable from FIG. 6, fibres with d/Λ smaller than 0.45 (correspondingto a filling fraction of 18%) will have V lower than 4.2 and are,consequently, endlessly single mode—meaning that the fibres can bedesigned with a large mode area and remain single mode. Fibres withlarger cladding features cannot be classified as endlessly single modeand they will become multi mode for large core sizes. It is, therefore,relevant to introduce the term “critical filing fraction”—or criticalhole diameter—as the largest possible filling fraction that can be usedfor endlessly single mode fibre. For the prior art fibres this criticalfilling fraction is, therefore, equal to 18%—or the critical air holediameter is d=0.45Λ. As an example of a fibre with hole size above0.45Λ, it is here chosen to look at a fibre with d/Λ=0.50. This fibrehas a cut-off around Λ/λ=3.5, hence for operation around 1.55 μm, themaximum dimensions for the fibre under single-mode operation that can betolerated is for Λ=3.5*1.55 μm or about 5.4 μm. This corresponds to acore diameter defined using 103 of around 10.8 μm and an inner corediameter defined using 104 of around 8.1 μm (the inner diameter beingequal to 2Λ−d for this type of fibre design).

In order to address the robustness of micro-structured fibres directly,FIG. 7 illustrates the macro-bending losses of prior art fibres withd/Λ=0.4, 0.5, and 0.6. The fibres have the same design as previouslyanalysed and the cladding feature spacing is Λ=4 μm—resulting in a corediameter from 103 of 8 μm and an inner core diameter from 104 of 6.4 μm,6.0 μm, and 5.6 μm, respectively for d/Λ=0.40, 0.50, and 0.60. Thebending losses are simulated for a bending radius of 6 cm (arepresentative value for presently used fibre drums for e.g. dispersioncompensating modules) and a method as described by Broeng et al. (seeOptical Fiber Technology, Vol. 5, pp. 305-330, 1999) has been employed.The figure shows that the large mode area endlessly single mode fibreswith d/Λ=0.4 is highly sensitive to bending losses—much more than fibreswith d/Λ larger than 0.5. The losses are higher than 10⁴ dB/m over thewhole wavelength range of interest (λ=0.3 μm to 2.0 μm) making itunusable for most practical applications as a large mode area fibre.Fibres with even smaller air holes than d/Λ=0.4 have even higher losses(these are not shown in the figure for reasons of clarity). While theprior art fibre with large cladding features d/Λ=0.6 is robust over awide wavelength range, it is, however, not single mode for wavelengthshorter than 2.6 μm (see FIG. 3) and this prior art fibre is, therefore,unusable in telecommunication systems. The figure gives us the importantinformation that the robustness—in terms of macro-bending losses—ofmicro-structured fibres is very sensitive with respect to air hole sizein a range around d/Λ=0.4 to 0.5. For smaller air holes, the fibres arehighly sensitive to bending losses, whereas for larger air holes thefibres are practically insensitive to bending losses. As those skilledin the art will recognise, the exact bending loss at a given wavelengthis depended on the exact feature spacing. As in the case of the cut-offproperties, the macro-bending loss properties may be scaled to a givenwavelength by scaling the fibre dimensions. Hence, for large mode areafibres, the high sensitivity around d/Λ=0.4 as discussed above is validfor the fibres in the wavelength range of interest.

Having demonstrated the principle limitation to presently known largemode area, micro-structured fibres, we turn to address the large-modearea, micro-structured fibres that have been demonstrated experimentallyin the literature. It is worth emphasizing that the experimentallyrealized large mode area, micro-structured fibres have, in fact, beeneven more bending sensitive than shown in the discussion above. FIG. 8shows a micrograph of a large-mode area fibre realize using the priorart fabrication method (picture taken from Birks et al. ElectronicsLetters, June 25, 34(13), pp. 1347-1348, 1998). The filling fraction isvery low (d/Λ around 0.12). The fibre has a core diameter of around 22.5μm and a cladding feature spacing, Λ, of around 9.7 μm. The fibre isexperimentally verified to be single mode at 458 nm (corresponding to anormalized frequency, Λ/λ, of around 22). It is important to notice thatthe fibre has a core diameter that is slightly larger than two times Λ,and the inner cladding features are elongated in directions surroundingthe core. These two characteristics of the fibre are attributed to thefabrication method (see e.g. Knight et al. Optical Materials, Vol. 11,pp. 143-151, 1999), where the core is formed from a solid cane at thepreform level. Due to collapsing holes in the cladding—most stronglypronounced near the core—the core effectively increases in size withrespect to the cladding pitch, compared to the core size at the preformlevel.

FIG. 9 illustrates the effective frequency, V, as a function ofnormalized frequency as in FIG. 6. The figure is taken from Knight etal. J. Opt. Soc. Am. A. 15(3), pp. 748-752, 1998, and it illustrates thetheoretical cut-off properties of prior art fibres with a design asshown in FIG. 1 (solid lines—as in FIG. 6) as well as experimentallyobtained result on prior art fibres with designs reminiscent of that ofFIG. 8 (circles and squares). Circles indicate fibres that are singlemode and squares indicate fibres that are multi mode. It is found thatfor the realized prior art fibres, the aforementioned critical fillingfraction is in fact very low, namely f less than 4% (d/Λ less than 0.2).This means that realized large mode area fibres in the prior art arevery sensitive to bending losses—see FIG. 7. The present inventors haverealized that the decrease in critical filling fraction of thefabricated, prior art fibres is attributed to the increase in corediameter relative to feature spacing compared to the design in FIG. 1,where the core diameter from 103 is exactly equal to two times thefeature spacing, Λ.

In order to understand the present invention, it is valuable to considermicro-structured fibres where the cladding is divided into (at least)two concentric cladding regions—an inner and an outer claddingregion—each having micro-structured features. FIG. 10 shows an exampleof a fibre according to the present invention. The fibre has circularcladding features 110 and 111 and the diameter, d_(i), of the innercladding features 110 are kept at a level around the critical diameterfor endlessly single mode operation, i.e. a diameter, d_(i), of around0.45Λ_(i), where Λ_(i) is the centre-to-centre spacing between two innercladding features. To improve the bending loss characteristics, theouter cladding features 111 have a larger diameter than the innercladding features—in this example the outer cladding features have adiameter, d_(o), of around 0.55Λ_(o), where Λ_(o) is thecentre-to-centre spacing between two outer cladding features. Throughadvanced numerical simulations, the present inventors have found thatfor silica-air micro-structures, the outer cladding features beingclosest to the inner cladding region should have a diameter that is notlarger than 0.6Λ_(o), for the fibres to remain single-mode at large coresizes. For d_(o)=0.55Λ_(o), fibres with a design as in FIG. 10 wherefound to remain single mode for core diameters (defined using 104) up to35 μm—and possibly larger cores may be made.

Depending of the refractive indices of the materials composing themicro-structured fibre (the core and cladding background materials andthe feature materials), the above-mentioned diameter ranges may vary.For silica and air structures, which are today the most employedmaterial choice for large-mode area micro-structured fibre, the presentinventors have found that the relevant range of diameters ared_(i)=0.40Λ_(i) to 0.45Λ_(i) for the inner cladding features andd_(o)=0.50Λ_(o) to 0.60Λ_(o) for those outer cladding features thatimmediately surround the inner cladding features. For other materials,such as for example polymers that provide a larger range of refractiveindices, these range may be broader, such as d_(i)=0.35Λ_(i) to0.50Λ_(i) and d_(o)=0.50Λ_(o) to 0.90Λ_(o).

The present inventors have also realised that further away from thecentre of the core, the outer cladding features may have a size that islarger than 0.6Λ_(o) while the fibre remains single-mode for large coresizes. It may be understood that the further away from the core theouter cladding features are, the smaller is their influence on thecut-off properties of the fibres. On the other hand, the increase indiameter of the outer cladding features compared to the inner claddingfeatures, results in a larger air filling fraction, that providesimproved macro-bending loss properties.

FIG. 11, illustrates fibres according to the present invention, wherethe outer cladding features increase in size away from the core centre110—as seen, outer cladding features 111 are larger than outer claddingfeatures 112. Outer cladding features that are positioned furthest awayfrom the core may have diameters, d_(o), of up to 0.9Λ_(o).

The present inventors have also realized that large-mode area fibres maynot be fabricated using a method where one (or more) solid cane(s) isused to form the core. In contrast to this, the present inventor haverealized that fibre designs, where the core diameter as defined from 104is smaller than 2Λ_(o)—preferably it is much smaller—are advantageousfor large mode area fibres with respect to robustness and single modeoperation. A detailed discussion of the advantages of such fibres shallbe presented in the proceeding sections, as well as a method of makingsuch fibres shall be described.

Another schematic example of a fibre according to the present inventionis illustrated in FIG. 12. The fibre has a core 120 with a diameter thatis larger than 4 μm, it has a micro-structured outer cladding with outercladding features 121 providing an outer filling fraction of more than18%, and it is guiding light in a single mode in the wavelength rangeconsidered for this invention, namely 0.3 μm to 2.0 μm. The innercladding features 122 have a non-circular shape. The inner features 122are oriented towards the centre of the core. The orientation of theelongated features may provide a bridging width, w, of around 0.55Λ_(i)that ensures that higher order modes are suppressed while a largermaximum dimension of the inner cladding features than 0.45Λ_(i) can beobtained. In this manner, an inner cladding region with an air fillingfraction of more than 18% can be obtained while the fibre is singlemode. The larger air filling fraction is advantageous in order to lowermacro-bending losses—as previously described. Intuitively, the advantageof this orientation of the features (usually air holes) compared tocircular-shaped features may be understood such that it enables light inhigher order modes to “escape” through the relatively large bridgingareas 123 of minimum width, w, that contain material with a refractiveindex comparable to the refractive index of the core region. In thisrespect, the worst orientation would be such that the bridging areaswere narrowed. This worst-case orientation is, however, exactly the caseof the prior art, experimentally demonstrated large mode area fibreshown in FIG. 8. In order for the bridging areas 123 to be as wide aspossible for a given filling fraction it is, therefore, optimum toorientate the cladding features 111 so as to point towards the corecentre—or in other words, an orientation towards the centre of the coreallows the largest possible filling fraction, when a certain bridgingwidth is required.

The bridging width, w, of a prior art fibre with a design as shown inFIG. 1 can be determined as w=Λ−d/Λ. Hence, the prior art fibres, wherea maximum d/Λ value of around 0.45 can be tolerated (corresponding to afilling fraction of 18%) in order for the fibre to be single mode, theminimum bridging width, w, is 0.55Λ. According to the present invention,it is, however, advantageous to have a filling fraction larger than 18%while keeping the bridging width around or smaller than 0.55Λ. While thewidth is here described in terms of the cladding feature spacing, it mayalso be appropriate to relate the width to the core diameter definedusing the ring 124 (similarly defined as 103) (especially fornon-periodic micro-structures). It is directly realized that thebridging areas should not be smaller than w=0.55ρ/2, where ρ is the corediameter defined using 124. This relation may also be stated in terms ofΛ_(i), where it would state: w equal to or larger than 0.55Λ_(i). Thepresent invention, therefore, includes preferred embodiments, where thefibres have bridging areas with width, w, of around 0.55Λ_(i) andfilling fractions larger than 18% for the outer cladding region andpreferably also for the inner cladding region. For a fibre core diameterof 4 μm or larger, this means that w should be around or larger than 1.1μm. It should be mentioned that the fabrication method presented in thepresent patent application allows fabrication of such fibres.

Since fibres according to the present application may have both largecladding features as well as an elongated shape of these, it may be moreappropriate to characterize the core size using an inner core diameteras described under the discussion of FIG. 1, i.e. an inner corediameter, ρ_(inner), defined using the ring 125 (similarly defined as104) of the core 102. For the prior art large mode area fibres, theinner diameter is significantly larger than Λ_(i) as well as Λ_(o),whereas fibres according to the present invention may well have innerdiameter less than Λ_(i). Therefore, in another aspect, the presentinvention includes fibres with Λ_(i) of at least 2 μm and an inner corediameter of at least 2 μm, but smaller than Λ_(i) and preferably alsosmaller than Λ_(o).

Other types of fibres according to the present invention may also berealised, such as fibres that have a core diameter that is larger than 4μm, but smaller than two times the outer cladding feature spacing,Λ_(o). The difference between such fibres and prior art fibres may beviewed as the inner cladding features being pushed towards the corecentre—resulting in a core diameter smaller than 2Λ_(o).

The present inventors have analysed a number of fibres according to thepresent invention with respect to cut-off properties and bending losses.Starting with the cut-off properties for a fibre with a design asillustrated in FIG. 12, FIG. 13 illustrates the effective frequency, V,as a function of normalized frequency. For fibres according to thepresent invention, the second order mode cut off is found to increase toeffective V value above 4.2. i.e. the fibres can be operated at highernormalized frequencies and remain single mode compared to prior artfibres. This is a strongly advantageous behaviour and it allowsrealization of fibres with larger mode areas than previouslypossible—for a given (large) filling fraction. In other words, thepresent inventors have found that the critical filling fraction of thenew fibres is increased compared to the prior art fibres. Hence, thisinvention provides endlessly single mode fibres with filling fractionslarger than 18%. As a result of the possibility of increasing thecritical filling fraction, it becomes possible to realize fibres withcore diameters of 4 μm or larger that are practically insensitive tobending losses in the wavelength range from 0.3 μm to 2.0 μm—inparticular for use in the wavelength range around 1.5 μm.

It should be noted that the effective V value at a given wavelength willnot be identical for a prior art fibre and a fibre according to thepresent invention although the fibres have identical core size andfilling fractions. The effective V value is given as V=k*ρ/2*NA, where kis equal to 2π/λ, ρ/2 is the core radius, and NA is the numericalaperture of the fibre. The above-mentioned difference in V value followsfrom the fact that the fibres have different relations between the corediameter and the cladding feature spacing, i.e. different featurespacing for a fixed core diameter that results in different effectivecladding indices. Hence, the numerical aperture of the fibres, which isstrongly dependent on wavelength, will not be identical, and, therefore,the effective V value will not be identical for a given core size and agiven wavelength.

Turning to the bending properties of fibres disclosed in the presentinvention, a fibre with a core diameter equal to 1.5Λ_(o) will be usedas example. For an absolute core diameter of 8 μm, the fibre will haveΛ_(o) equal to 5.3 μm. To compare this with a prior art fibre with acore diameter of 2Λ (Λ_(i)=Λ_(o)=Λ), this would require Λ=4 μm. Whilethis for identically sized cladding features relative to Λ_(o) wouldrender the fibres according to the present invention more sensitive tobending losses, the advantage of the fibres disclosed in thisapplication, is that their critical filing fraction is larger than forthe prior art fibres. Hereby, the new fibres may be designed with airfeature sizes that cannot be tolerated in the prior art fibres. For thespecific example, this means that single-mode fibre with core diameterof 8 μm and a feature size, d_(o)/Λ_(o), of about 0.55 can be realized,whereas the prior art single-mode, large-mode area fibres can only berealized with a feature size, d/Λ, of about 0.45. Therefore, althoughΛ_(o) for a fibre according to the present invention with a givenabsolute core size may be larger compared to Λ for a prior art fibrewith identical core size, the increased critical air filling fractionallows the here-disclosed fibre to be designed with a larger d_(o)/Λ_(o)and consequently to be more robust than the prior art fibre.

To demonstrate the advantage of fibres according to the presentinvention, the bending losses should be compared for a prior art, singlemode fibre with d/Λ=0.4 and core diameter of 2Λ, and a single-mode fibreaccording to the present invention with d/Λ=0.6 and core diameter of1.5Λ. For a proper comparison with respect to large mode area fibres,both fibres should have identical spotsize. As a generally acceptedspotsize for micro-structured fibres has not yet been defined, thecomparison is made for fibres with identical core diameter—and anabsolute core diameter of 8 μm has been chosen. Hence, the prior artfibre has Λ=4.0 μm and d=1.6 μm, and the fibre according to thisinvention has Λ_(o)=5.3 μm and d_(o)=3.2 μm. FIG. 14 presentsschematically the details for such a comparison. As seen from thefigure, the fibre according to the present invention is superior to theprior art single-mode, large mode-area fibre. Again, it should beemphasized that, the prior art fibre with d/Λ=0.6 that appears to besuperior to the fibre according to the present invention, is, in fact,unusable for telecommunication applications, as it is multi mode forwavelengths shorter than 2.6 μm.

Fibres according to the present invention are characterized by cut-offproperties that are different to those of prior art micro-structuredfibre, in the way that single mode fibres with relatively large modeareas and filling fraction above 18% can be realized. The increasedfilling fraction provides increased possibility of creating strongwaveguide dispersion in single mode, micro-structured fibres. Therebythe present invention provides new means for tailoring the dispersionproperties of large mode area, single mode fibres. The present inventorshave realised that fibres according to the present invention may be ofparticular interest for use in applications such as long-distancetransmission, dispersion compensation, dispersion slope compensation andhigh power delivery over a broad range of visible wavelengths.

In order to fabricate the fibres of the present invention, a preformmade purely from a stack of pure capillary tubes can be used—seeschematic FIG. 15. At the preform level, the tubes 150 in the claddingregion may be sealed in the top end, while one tube in the centre iskept open 151. During fibre drawing the central—open—tube 151 willcollapse completely and provide a solid core 152 with an innerdimension, ρ_(inner), significantly smaller than two times the innercladding feature spacing. This method also provides nearest-corefeatures 153 around the core region, that have an orientation towardsthe core centre, since the collapsing core-tube 151 will deform thesurrounding tubes as desired.

FIG. 16 shows yet another example of a fibre according to the presentinvention. The fibre has three relatively large inner features 160surrounding the core 161 and smaller, more densely spaced outer features162 in the outer part of the cladding. The features 160 have a spacingΛ_(i) and the features 162 have a spacing Λ_(o). The core diameterdefined using the ring 163 is for this specific fibre significantlybelow 2Λ_(i)—more precisely it is around 1.15Λ_(i). According to thepresent invention, the second order mode of this fibre will besignificantly suppressed, and the fibre can be designed with very largefeatures 161, while it remains single mode. In preferred embodiments,the inner features 160 have, therefore, a diameter that is significantlyabove 0.45Λ_(i), such as 0.6Λ_(i). For fibres with a core diameter of atleast 4 μm, this renders features 160 with a dimension of at least 1.5μm, such as at least 2.0 μm.

The present inventors have realised yet another manner of improvingmicro-structured fibres through the use of outer lying regions thatmechanically can favour specific bending directions. FIGS. 17 to 21shows schematic illustrations micro-structured fibre with non-circularand non-equilateral polygonal shapes of the outer part of the fibres.This provides new means for controlling bending induced properties suchas macro-bending losses, dispersion and cut-off, etc. In FIG. 17 is seenan outer cladding region 170 for obtaining a preferred bending direction171 (or the direction 180-degrees opposite 171). The present inventorshave realised that any given micro-structured fibre may be improved withrespect to lower macro-bending losses by having the fibre bend inpreferred directions. The figure shows a fibre with an outer claddingregion 170—which may be composed of silica, polymer, or various othermaterials, e.g. materials with micro-structures—that has an axis ofsymmetry defined by the shortest dimension (parallel to and coincidingwith 171) of the outer cladding region 170. The mode field willexperience less leakage under bending in direction 171 than if the fibrehad been bend in direction 172 as the field will more easily escape indirections between two low-index features. Hence, the specificorientation of the outer cladding region 170 preferably depends on thespecific micro-structured. In a preferred embodiment, the outer region170 is oriented such that it has a smallest dimension with a smallestaxis that substantially coincides with an axis through centres of twoopposite, innermost cladding features 173. In other words, centres oftwo innermost, opposite cladding features 173 are substantiallypositioned on the smallest axis.

The present inventors have further realised that if the claddingfeatures are large enough to provide bending insensitivity of thefundamental mode at any bending direction for realistic bending radil,it will be advantageous to bend the fibre in the direction 182 in orderfor higher order modes to be stripped off through macro-bendinglosses—see FIG. 18. In this manner it will be possible to expand thesingle-mode range of a given micro-structured fibre. In this latterpreferred embodiment, the smallest dimension of the outer claddingregion 180 should be oriented along the direction 182. This preferredembodiment may also be described by stating that the smallest dimensionof the outer cladding region 180 should have an orientation thatcoincides with an axis through two bridging areas positioned oppositeeach other around the core centre. In other words, centres of twoinnermost, adjacent cladding features 183 are substantially equidistantto an axis (coinciding with 182) defined by the smallest dimension ofthe outer cladding region 180.

Other shapes than elliptical, such as rectangular, may also provide thesame type of functionality as described above, and therefore beadvantageous. An example of a rectangular shape of the outer cladding isshown schematically in FIG. 20. Another preferred shape is shownschematically in FIG. 21.

In yet another aspect relating to macro-bending properties ofmicro-structured fibres, the present inventors have realized that it ispossible to control the direction of radiation of a guided mode duringfibre bending. This may e.g. be performed using the types ofnon-circular and non-equilateral-polygonal shaped cladding regions asdescribed previously, or it may be performed by specially designedarrangement of micro-structured features in the cladding (and/or core)region. Considering first the types of non-circular outer claddingfeatures that was described above, the radiation will predominantly takeplace in two, opposite directions, namely from the core centre and alongthe axis defined from the smallest dimension of the outer cladding. If,however, the cladding features in, e.g., one half of themicro-structured part of the cladding region are positioned so as toprovide a more dense structure, then the radiation in one of theafore-mentioned directions will be stronger than in the other.Therefore, in another aspect, the present invention relates tomicro-structured fibres with different micro-structures in differentdirections from the core centre. An example of a preferred embodiment ofa fibre with such bending properties is illustrated schematically inFIG. 22. The fibre has micro-structures with different characteristicsin different directions from the core centre. The fibre has claddingfeatures 220 that are smaller in size in approximately half themicro-structured part of the fibre. Also smaller segments of themicro-structured part of the fibre may contain features with differentcharacteristics, and still provide the functionality of directionaldependent radiation. The different micro-structured parts of the fibremay be obtained in various ways, by different feature sizes (as in FIG.22), different feature arrangements, different periodicity, etc. Fibresaccording to this invention may find possible applications as pumpingcomponents, where the fibre is coiled around a medium that should absorblight radiated from the coiled fibre. Various types of applications,where a micro-structured fibre is used to provide optical energy to asingle or more media are also covered by the present invention.Typically, the fibre will provide low loss guidance of electromagneticenergy (light) from a source to a specific medium. By coiling the fibreaccording to the present invention around the medium with a specificmechanical bending determined by an outer non-circular cladding region,the energy of the guided mode can be radiated to the media by the use ofmicro-structures with different periodicity in different directions fromthe core. Hence, the fibre should be designed to radiate its energy indirection of the inside of the fibre coil. This property is not possibleusing conventional fibres. The present inventors have further realisedthat such a property can be obtained also in micro-structured fibresguiding light by photonic bandgap effects (where a full periodicity of amicro-structured cladding region can be tolerated). The presentinvention, therefore, also covers the afore-mentioned fibre pumpingapplications, where a photonic bandgap guiding micro-structured fibre isused.

The present inventors have further realised that fibres with a design asschematically outlined in FIG. 22 may be used to obtain fibre dispersionthat Is strongly dependent on the fibre bend radius. The fibre may havevery large holes 221—significantly above the critical size for prior artfibres—and be single mode, if the smaller holes 220 are equal to or lessthe critical size (f<18%). This reason for the fibre being single modeis that higher order modes will leak out the side of the fibre mainly indirection 222—even under straight-fibre operation—whereas thefundamental mode will not leak. For applications where fibre dispersionare of importance—such as e.g. for dispersion compensation or dispersionslope compensation—the present inventors have realised that the type offibre shown in FIG. 22 may be advantageous. Most importantly, thepresent inventors have realised that the dispersion in fibres heredisclosed can be controlled to a larger degree than standard fibresthrough the fibre bending radius. For a straight fibre, the propagatingmode will experience specific surroundings (core and claddingstructures)—and, therefore, a specific dispersion. If the fibre is bendat a certain radius, the mode field will be differently distributed inthe core/cladding region, and the fibre will experience a differentdispersion. Hence, a tuneable dispersion becomes feasible, or a certain,fixed dispersion can be reached by bending the fibre to a certainradius. In order to avoid the mode field from leaking away during thefibre bend, it is advantageous that the fibre is bend in the direction222 of the smaller features 220, since the mode field will thenexperience the larger features 221 more strongly (increasing thewaveguide dispersion) as well as these larger features 221 will ensurethat the mode field is not radiated away. Hence, the fibre can be maderobust and single mode for large mode areas and have special dispersionproperties unattainable in conventional as well as prior artmicro-structured fibres. In order to ensure that the fibre is bend inthe direction 222 it will again be an advantage to utilize anon-circular outer cladding region as previously discussed.

As a further preferred embodiment of a fibre with a substantiallyelliptical outer cladding region, it is preferred to have a fibre withsmaller features 230 positioned as in FIG. 23 in direction to towardsthe largest dimension of the outer cladding region and larger features231 positioned in directions towards the smallest dimension of the outercladding region. In this manner, a straight fibre will only guide asingle mode, even for very large features 231 (f>18%), when the smallerfeatures are relatively small (f<18%). The reason for this is that ahigher order mode will radiate away in the directions 232 and 233. Thefibre in FIG. 23 will furthermore be relatively insensitive tomacro-bending losses, since the outer cladding region ensures that thefibre will predominantly be bend in the directions 234 and 235. Forbending in these directions, the larger holes 231 will provide a strongconfinement of the guided mode. The fibre in FIG. 23 can, therefore, berealised with a relatively large mode area, it can be single mode and itcan be insensitive to macro-bending losses—all at the same time.

In order to make an outer cladding that ensures bending in one or twopreferred directions various methods and techniques can be used. Theouter cladding region may be made already at the preform level—forexample by fusing two (smaller) glass rods to the preform as indicatedin FIG. 21. Alternatively, the outer cladding region may be applied tothe fibre after drawing—such as e.g. through various types of coating,jacketing, cabling etc.

For all aspects of the present invention, it may be advantageous to havea fraction or all of the core area doped to provide a special refractiveindex profile for tailoring dispersion properties of the fibres, or forexample to provide a higher refractive index than the refractive indexof the background material, which may be advantageous for splicing ofmicro-structured fibre to conventional fibres or to othermicro-structured fibres, etc. During splicing the features maycollapse—leaving a solid background material that is incapable ofproviding guidance. Hence, to have a part of the core region doped withe.g. Germanium may provide improvements with respects to splicinglosses. In a preferred embodiment, a part of the core is doped toprovide a higher refractive index in a central part of the core, with adiameter of less than 2 μm, such as less than 1 μm.

Finally, it should be emphasised that both periodic and non-periodicarrangements of features can be used in all aspects of the presentinvention—as well as combinations of these types of arrangements invarious regions of a single fibre.

1. A micro-structured optical fibre for guiding light at an operatingwavelength, said optical fibre having an axial direction and a crosssection perpendicular to said axial direction, said optical fibrecomprising: a core region, an inner cladding region surrounding saidcore region, said cladding region comprising a multiplicity of spacedapart inner cladding features that are elongated in the fibre axialdirection and disposed in an inner cladding material, said innercladding features having a smallest cross-sectional dimension,d_(i,min), and a largest cross-sectional dimension, d_(i,max), andhaving a centre-to-centre spacing, Λ_(i), between two neighbouring innercladding features, an outer cladding region surrounding said innercladding region, said outer cladding region comprising a multiplicity ofspaced apart outer cladding features that are elongated in the fibreaxial direction and disposed in an outer cladding material, at leastpart of said outer cladding features having a smallest cross-sectionaldimension, d_(o,min), and having a centre-to-centre spacing, Λ_(o),between two neighbouring inner cladding features, wherein d_(i,min) isin the range from 0.35Λ_(i) to 0.50Λ_(i), d_(o,min) is in the range from0.50Λ_(o) to 0.90Λ_(o), d_(i,min) is smaller than d_(o,min), and Λ_(i)is larger than 2.0 μm.
 2. An optical fibre according to claim 1 whereind_(i,min) is in the range from 0.40Λ_(i) to 0.45Λ_(i).
 3. An opticalfibre according to claim 1 wherein d_(o,min) is in the range from0.50Λ_(o) to 0.60Λ_(o).
 4. An optical fibre according to claim 1 whereind_(i,max) is in the range from 1.0d_(i,min) to 2.0d_(i,min).
 5. Anoptical fibre according to claim 1 wherein Λ_(i) is in the range from0.3Λ_(o) to 3.0Λ_(o), or Λ_(i) is about equal to Λ_(o).
 6. An opticalfibre according to claim 1 wherein Λ_(i) is larger than 2.5 μm, such aslarger than 3 μm, such as larger than 5 μm, such as larger than 10 μm,or such as larger than 25 μm.
 7. An optical fibre according to claim 1wherein d_(o,min) is between 10-50% larger than d_(i,min).
 8. An opticalfibre according to claim 7 wherein d_(o,min) is between 10-20% largerthan d_(i,min).
 9. An optical fibre according to claim 8 whereind_(o,min) is between 10-15% larger than d_(i,min).
 10. An optical fibreaccording to claim 1 wherein the core region has a geometrical indexN_(coge), the inner cladding region has a geometrical index N_(ige), theouter cladding region has a geometrical index N_(oge), andN_(coge)>N_(ige)> or =N_(oge) for light guided at said operatingwavelength.
 11. An optical fibre according to claim 1 wherein the coreregion has an effective refractive index N_(coef), the inner claddingregion has an effective refractive index N_(ief), the outer claddingregion has an effective refractive index N_(oef), and N_(coef)>N_(ief)>or =N_(oef) for light guided at said operating wavelength.
 12. Anoptical fibre according to claim 1 wherein the outer cladding featuresbeing nearest neighbours to the inner cladding region all have asmallest cross-sectional dimension given by d_(o,min).
 13. An opticalfibre according to claim 1 wherein a majority or all of the outercladding features have a smallest cross-sectional dimension given byd_(o,min).
 14. An optical fibre according to claim 1 wherein at leastpart of the inner cladding features in the cross-section have anelongated shape, with the largest cross-sectional dimension d_(i,max)being oriented in a direction substantially towards the centre of thecore region.
 15. An optical fibre according to claim 1 wherein thecladding features are periodical features.
 16. An optical fibreaccording to claim 1 wherein all or at least a part of the claddingfeatures are non-periodical features.
 17. An optical fibre according toclaim 1 wherein the core region is a substantially solid core made of acore material.
 18. An optical fibre according to claim 17 wherein theeffective refractive index of the core N_(cief) and/or the geometricalindex of the core N_(coge) is substantially equal to the refractiveindex of the core material.
 19. An optical fibre according to claim 1wherein the optical fibre is dimensioned to guide light at an operatingwavelength selected from wavelengths in the range of 300 nm to 1600 nmin a single mode.
 20. An optical fibre according to claim 19 wherein thecore region comprises one core feature or a multitude of spaced apartcore features.
 21. An optical fibre according to claim 20 wherein thecore features have a cross-sectional dimension being smaller than thesmallest cross-sectional dimension of the inner cladding features. 22.An optical fibre according to claim 20 wherein the core features arevoids.
 23. An optical fibre according to any of claim 22 wherein thevoids of the core region contain air, another gas, or a vacuum.
 24. Anoptical fibre according to claim 22 wherein at least part of or all ofthe core features and/or the cladding features are voids containingpolymer(s), a material providing an increased third-order non-linearity,photo-sensitive material, or a rare earth material.
 25. An optical fibreaccording to claim 20 wherein the core region contains core featureswith a non-circular symmetric shape in the fibre cross-section.
 26. Anarticle comprising an optical fibre according to claim 19 wherein thecore region comprises a core feature with a higher refractive index thanany material surrounding the core feature.
 27. An article comprising anoptical fibre according to claim 26 wherein the core feature has adimension of less than 2 μm, such as less than 1 μm.
 28. An opticalfibre according to claim 1 wherein the optical fibre is dimensioned toguide light at an operating wavelength about 1550 nm in a single mode.29. An optical fibre according to claim 1 wherein the refractive indexof the core material is lower than the refractive index of the innerand/or outer cladding material.
 30. An optical fibre according to claim1 wherein the refractive index of the core material is substantiallyequal to the refractive index of the inner and/or outer claddingmaterial.
 31. An optical fibre according to claim 30 wherein the corematerial and the inner cladding material are made of the same material.32. An optical fibre according to claim 30 wherein the core material anthe outer cladding material are made of the same material.
 33. Anoptical fibre according to claim 1 wherein the core material is silicaor polymer.
 34. An optical fibre according to claim 1 wherein the innerand/or outer cladding material is silica or polymer.
 35. An opticalfibre according to claim 1 wherein the cladding features are voids. 36.An optical fibre according to claim 35 wherein the voids of the claddingregions contain air, another gas, or a vacuum.
 37. An optical fibreaccording to claim 1 wherein the core and/or any of the claddingmaterials contains polymer(s), are material(s) providing an increasedthird-order non-linearity, are photo-sensitive material(s), or are rareearth material(s).
 38. An optical fibre according to claim 1 whereinsaid fibre is dimensioned to guide light of an operating wavelength intwo substantially, non-degenerate polarization states.
 39. An opticalfibre according to claim 38 wherein the fibre is characterized by abirefringence of at least 10⁻⁶, such as of at least 10⁻⁴, such of as atleast 10⁻³.
 40. An optical fibre according to claim 1 wherein the shapeof the core region deviates substantially from a circular shape in thefibre cross-section.
 41. An optical fibre according to claim 1 whereinthe shape of the core region deviates substantially from a quadraticshape, a hexagonal shape, or a higher order polynocnial shape in thefibre cross-section.
 42. An optical fibre according to claim 1 whereinthe shape of the core region is substantially rectangular in thecross-section.
 43. An optical fibre according to claim 1 wherein thecore region and/or the cladding regions have substantially a 180 degreerotational symmetry in the fibre cross-section.
 44. An articlecomprising an optical fibre according to claim 1 wherein the fibre iscoiled with a bend radius of 20 cm or less, such as 15 cm or less, suchas 10 cm or less, such as 6 cm or less, such as 1 cm or less.
 45. Anarticle according to claim 44 wherein the fibre is used for dispersioncompensation or dispersion slope compensation.